1. Field of the Invention
This invention relates to electronic theft detection systems and more particularly it concerns novel arrangements for maintaining the vicinity of a theft detection checkpoint substantially uniformly filled with an electromagnetic interrogation field.
2. Description of the Prior Art
The present invention is especially suitable for use in conjunction with electronic theft detection systems of the type described in U.S. Pat. Nos. 3,493,955 and 3,500,373. In both systems, each of the articles to be protected from theft has a electronic responder circuit attached to it. This circuit may be concealed in a wafer like element which may also serve as a price label or the like for the protected article. The articles are maintained in an enclosure having limited egress and checkpoints are set up at each egress. A transmitter is provided at the checkpoint to transmit an interrogation signal and receiver means are provided to note any response produced by the interaction of a wafer responder circuit with the transmitted signal field in the vicinity of the checkpoint. In the case of the systems described in U.S. Pat. No. 3,493,955, the wafer responder circuits respond to the transmitted interrogation signal, which is at first frequency, to produce a response signal at a second frequency. The receiver means are tuned to detect this second frequency.
In the case of the system described in U.S. Pat. No. 3,500,373, the wafer responder circuits are resonant circuits tuned to resonate at the transmitted interrogation frequency. When these wafer responder circuits are brought into the transmitted interrogation signal field they absorb some of the transmitted energy. The receiver means monitors the transmitted signal, which changes in amplitude due to this absorption. In order to maximize sensitivity the transmitter of this system produces an output frequency which sweeps cyclically over a given range which includes the resonant frequency of the wafer responder circuits. This causes a series of responses in the form of impulses which occur at a repetition rate corresponding to the frequency sweep rate.
The ability of a responder circuit to function effectively in any electronic theft detection system depends upon the degree to which the interrogation field is incident upon the responder circuit. Since these responder circuits are generally in flat wafer-like form, they exhibit different degrees of sensitivity depending upon their orientation with respect to the interrogation antenna. In order to accomodate the different attitudes which responder devices may assume when carried through a checkpoint there has been developed a plural antenna system comprising at least two antennas positioned at right angles to each other at the checkpoint. The two antennas are energized simultaneously so that as a responder circuit is turned away from one interrogation antenna it turns toward the other interrogation antenna so that sensitivity is maintained. This plural interrogation antenna system is shown and described in U.S. Pat. No. 3,493,955.
The present invention is directed to a somewhat related but different problem than the orientation problem described above. It has been found that even plural interrogation antenna systems produce an uneven electromagnetic field energy distribution throughout the vicinity of a checkpoint. This occurs as a result of the additive and subtractive effects of the fields produced by the different antennas. As a result of these effects there are developed "dead zones" or regions of minimal electromagnetic field intensity. If a responder circuit is caused to follow a path through the dead zone regions of a checkpoint only minimal interaction will occur between the interrogation signal and the responder circuit and it is possible that the passage of the responder circuit through the checkpoint will not be detected.